The present invention relates to an evaluation method and an evaluation apparatus for evaluating battery safety, and to a battery whose safety indices have been determined with the evaluation method and the evaluation apparatus. In particular, the invention relates to an evaluation method and an evaluation apparatus for evaluating battery safety at the time of occurrence of an internal short circuit, and a battery whose safety indices have been determined with the evaluation method and the evaluation apparatus.
In the case of evaluating the safety of lithium ion secondary batteries and alkaline dry batteries, for example, one of the items to be evaluated is the exothermal behavior at the time of occurrence of an internal short circuit. The content of the test methods for examining such a behavior is prescribed, for example, by the UL standard for lithium batteries (UL 1642) and a guideline (SBA G1101-1997 “Guideline for Safety Evaluation on Secondary Lithium Cells”) from The Battery Association of Japan. Such tests are utilized for the development of various batteries (see e.g., Laid-Open Patent Publication No. Hei 11-102729. In particular, paragraph [0048]).
One of these test methods is the nail penetration test in which a nail is driven through batteries from the side. In this test method, the nail that has penetrated through batteries creates a short circuit portion in the interior of the batteries, and a change, for example, in the battery temperature and the battery voltage that was caused by the Joule heat generating at that short circuit portion is observed. Another test method is the crush test in which batteries are physically deformed using a round bar, a square bar or a flat plate, for example. In this test method, the above-mentioned physical deformation causes a short circuit in the interior of the batteries, and a change, for example, in the battery temperature and the battery voltage that was caused by the short circuit is observed.
The present inventors analyzed the exothermal behavior at the time of occurrence of an internal short circuit with regard to the above-described nail penetration test and crush test. According to the results of the analysis, it was evident that plural locations in the interior of each battery simultaneously experienced a short circuit in these tests. Further, in these tests, it is essential to greatly deform or damage the constituting elements of the electrode group, such as electrodes and a separator, intentionally. As a result, these tests cannot evaluate the battery safety with regard to internal short circuits that occur locally.
Each of the above-described tests examines the exothermal behavior in an internal short circuit that occurs due to a physical cause and that also occurs in a very extreme manner. However, in practice, the battery safety becomes a problem in an internal short circuit that occurs in a manner that is difficult to recognize visually, and such an internal short circuit often occurs through a chemical process. For example, conductive fine foreign matter enters into the interior of batteries, and that foreign matter is dissolved by the electrolyte and then deposited to cause such an internal short circuit. In that case, the thermal behavior cannot be examined by the above-described test methods.
In addition, the safety level, including, for example, “There is no heat generation” or “There is some heat generation” at the time of occurrence of an internal short circuit through the above-described chemical process cannot be known by these tests. It is very important to know such a safety level in connection with the applications of batteries (for example, a low level of heat generation is required for the mobile phone application). For this reason, there is a great demand for an evaluation method that can determine the safety level at the time of occurrence of an internal short circuit for each type of batteries.
Foreign matter may enter into the interior of batteries not only during manufacture of batteries, but also after batteries went onto the market as products. Particularly, from the viewpoint of preventing accidents, it is very important to know the exothermal behavior in the case where entrance of foreign matter has caused an internal short circuit in batteries after they went on the market.
The present invention was made in view of the foregoing problems, and it is an object of the invention to provide an evaluation method and an evaluation apparatus for batteries with which it is possible to know the safety level in the case where foreign matter enters into the interior of batteries, and has caused an internal short circuit through a chemical process. It is another object of the present invention to provide a battery whose safety level is determined by the above-described evaluation method and evaluation apparatus.